The effectiveness of the material used as the abrasive grit in grinding wheels varies with the grinding conditions and the particular material being ground.
For example, cofused alumina zirconia abrasive made according to the teachings of U.S. Pat. No. 3,891,408 is very effective in grinding wheels for grinding 304 stainless steel and for grinding carbon steels at high metal removal rates, but is less effective, as compared to fused alumina, at slow grinding conditions.
The explanation for the different effectiveness of a given abrasive under differing grinding applications is often not clearly understood, but is related to the chemical composition, microstructure, and the related physical properties of the abrasive such as hardness, fracture toughness, impact strength, and thermal properties. Lack of such understanding makes difficult the prediction of the effectiveness of a given abrasive for a particular application in the absence of prior experience. Relatively new types of sintered abrasives made from dried alumina gels by sintering have been developed. One such type is that disclosed in U.S. Pat. No. 4,314,827. In particular, the embodiment of that invention disclosed in examples 19 to 29 (containing 4 to 8% MgO) is being produced on a commercial basis. Another alumina abrasive grit produced from a gel is disclosed is E.P.O. Application No. 85100506.6. That abrasive grit can be essentially pure alumina or may also contain MgO, and is made from gels which have been seeded with submicron alpha alumina particles.
The present invention relates to the combination, in grinding wheels, of the sintered alumina gel type abrasive with cofused alumina-zirconia abrasive.